Bit twiddling: What does warning CS0675 mean?

From the sublime level of continuation passing style we go back to the mundane level of twiddling individual bits.int i = SomeBagOfBits();ulong u = SomeOtherBagOfBits();ulong result = u | i; // combine them together Whoops, that's an error. "Operator | cannot be applied to operands of type int and ulong." There are bitwise-or operators defined on int, uint, long and ulong, but none between int